As background art, "A method of controlling an electric power for discharge processing" disclosed in Japanese Patent Publication No. HEI 2-34732 is shown in FIG. 12 of the attached drawings. In FIG. 12, the reference numeral 17 denotes a discharge detecting unit for detecting that discharge occurs at a location between an electrode and a workpiece; 18, an output signal line of the discharge detecting unit 17; 19, a logic circuit for selecting driving of switching elements 21.about.25; 20a.about.20e, AND circuits; 26.about.30, resistances for selecting values of processing current, which are connected in series to the switching elements; 31.about.35, outputs from the logic circuit 19. A conventional power-source apparatus for discharge processing has been constructed as described above.
Operation of the above-described circuit will next be described with reference to FIG. 13 and FIGS. 14A and 14B. At the start of processing, the logic circuit 19 outputs an ON-signal to all of the switching elements 21.about.25. At this time, as shown in FIG. 13, a pulse signal P issues from an oscillator 3. The switching elements 21.about.25 are turned ON in synchronism with the pulse signal P. A voltage waveform 36 as processing voltage illustrated in FIG. 14A and a current waveform 37 illustrated in FIG. 14B are supplied, to a location between poles 5.
Next, when discharge occurs between the poles 5, the discharge detecting unit 17 outputs a discharge detecting signal D shown in FIG. 13. The logic circuit 19 turns OFF the switching elements 23.about.25, while the switching elements 21 and 22 remain turned ON. In this example, the resistances 26 and 27 are an internal impedance in a power source for producing original predetermined discharge current. That is, no-load time until voltage is applied to the location between the poles 5 so that discharge occurs is controlled such that all of the switching elements 21.about.25 are turned ON, and the internal impedance in the power source is brought to a value sufficiently small as compared with the interpole impedance.
When the discharge occurs between the poles 5, the switching elements 21.about.25 are so controlled in switching as to produce the original predetermined discharge current, and the internal impedance in the power source is controlled to a predetermined value.
As described above, the conventional method of controlling the power source for discharge processing is a system in which the switching elements are turned ON/OFF, and the resistances are switched, to supply the discharge current.
In addition to the above, as reference literatures relating to the present invention, there are "A power source for discharge processing" disclosed in Japanese Patent Laid-Open No. HEI 3-208521, "A waveform control unit for a discharge processing apparatus" disclosed in Japanese Patent Laid-Open No. HEI 3-73220, and "A power source apparatus for discharge processing" disclosed in Japanese Patent Laid-Open No. HEI 3-55117.
Since the above-described conventional method of controlling the power source for discharge processing has been constructed or arranged as described above, there are the following problems. That is, first, much of the electric power supplied from the power source is consumed by the resistances so that a power source of large electric power capacity is required. Thus, since the resistances generate heat, a cooling unit is required, that is, an arrangement is large-sized, so that small-sizing of the apparatus is impeded. Further, since electric power efficiency is low, waste electric power is consumed so that a running cost will increase or rise. Furthermore, since there are many cases where the only power source used will transform alternating power to a voltage that is then rectified and is smoothed, the processing current largely varies or fluctuates when the voltage of the alternating power source fluctuates so that processing accuracy is deteriorated. Moreover, fourthly, when processing is made or executed by a power-source apparatus for a discharge processing machine, of a direct-current output, micro cracks occur in a processed surface of the workpiece, so that the processing quality is deteriorated, and electrolytic corrosion occurs in the electrode, the workpiece and the like. Finally, when short-circuiting occurs between the poles, the processing current flows more than a set value so that there is a fear that the electrode and the workpiece are damaged.